The present invention relates generally to the field of cleaning or treating miniature electromechanical device surfaces with cryogenic impingement sprays. More specifically, the present invention relates to the field of environmental control for performing cryogenic spray cleaning processes. Conventional precision cleaning processes using cryogenic particle impingement sprays such as solid phase carbon dioxide require control of the atmosphere containing a treated substrate to prevent the deposition of moisture, particles and other such contaminants onto cleaned surfaces during and following cleaning treatments.
Environmental control is required because of localized atmospheric perturbations created by the low temperatures and high velocities which are characteristic of these impingement cleaning sprays.
For example, snow particles having a surface temperature of xe2x88x92100 F. and traveling through the space between the spray nozzle and substrate are continuously sublimating in transit and upon impact with a substrate surface. This rapidly lowers local ambient atmospheric temperaturexe2x80x94causing the contaminants contained therein to condense or xe2x80x9crain-outxe2x80x9d of the local atmosphere and onto treated substrate surfaces during or following spray treatments. Moreover, the cleaning spray stream exhibits lower internal pressure than the 3 surrounding atmosphere (Bernoulli Principle) and creates venturi currents adjacent to the flow of the stream. These venturi currents cause the local atmosphere surrounding the stream to collapse into the spray stream above the substratexe2x80x94thus entraining and delivering a mixture of cleaning spray and atmospheric constituents to the substrate. Finally, static charge build-up and accumulation are common to cryogenic sprays due the dielectric and tribocharging characteristics. This presents two problemsxe2x80x94potential device damage from electrostatic overstress (EOS) or discharge (ESD) events and attraction of atmospheric contaminants to treated substrates via electrostatic attractive forces.
Micro-environmental control technology is well established and many techniques have been developed over the years to isolate a process, a substrate or a worker. The purpose of isolation can include protecting workers from toxic chemicals, protecting clean rooms from particles, or protecting delicate processes and substrates from the outside environment, among many others.
There are many examples of techniques to control thermal and electrostatic effects during cryogenic impingement sprays using secondary heated or ionized jets or sprays above the substrate surface and delivered either independently or as a component of the cryogenic spray have been used commercially. In U.S. Pat. No. 5,409,418 and U.S. Pat. No. 5,354,384, both teach direct heated or ionized gas impingement techniques and apparatus for heating, purging and deionizing substrate surfaces.
""384 teaches the use of a heated gas such as filtered nitrogen to provide a pre-heat cycle to a portion of a substrate prior to snow spray cleaning the same portion of said substrate, and a post-heat cycle of same said portion following snow cleaning. This approach relies on xe2x80x9cbanking heatxe2x80x9d into the substrate portion prior to cryogenic spray cleaning by delivering a heated gas stream to a portion of substrate to prevent moisture deposition and adding heat from a heated gas following cryogenic spray treatment. The ""384 invention is primarily useful for removing high molecular weight materials such as waxes and adhesive residues from surfaces by partially melting or softening them prior to spray treatmentxe2x80x94in essence weakening cohesive energy. However, this approach does not work well for most substrate treatment applications. This is because many materials, or the portions thereof, being cleaned have low thermal conductivity and low mass or because highly thermal conductive materials rapidly lose heat to the sublimating snow during impact. This creates localized cold spots on even a mostly hot bulk substrate, which is the phenomenon for many substrates and surfaces being treated. Examples include ceramics, glasses, silicon and other semi-conductor materials, as well as most polymers. In addition, many electromechanical devices being cleaned are very smallxe2x80x94providing no appreciable mass for storing heat. Examples include photodiodes, fiber optic connectors, optical fibers, end-faces, sensors, dies, and CCD""s, among many others.
Most significantly, directing a heating spray, or any secondary fluid for that matter, directly at or incident with the substrate surface to be cleaned prior to, during and/or following cryogenic cleaning spray treatments causes the entrainment, delivery and deposition of atmospheric contaminants as discussed above. This necessitates housing the cryogenic spray applicator, substrate and secondary gas jets in large, bulky and complex environmental enclosures employing HEPA filtration and dry inert atmospheres such as taught for example in U.S. Pat. No. 5,315,793, which teaches a fully enclosed environmental chamber containing a snow spray applicator, automation, and thermal control system.
In the ""418 invention, an apparatus is taught for surrounding the impinging cryogenic spray stream with an ionized inert gas. Using this invention, it is proposed by that surrounding a stream of solid-gas carbon dioxide with a circular stream of ionized gas and applying the two components to the substrate simultaneously controls or eliminates ESD at the surface during impingement. However, as also with ""384 invention above, the ""418 secondary stream entrains, delivers and deposits atmospheric contaminants upon the substrate surfaces being treated. Moreover, contact of the ionizing gas with the stream prior to contact with the surface rapidly eliminates ion concentrationxe2x80x94highly degrading the performance of such an approach to controlling ESD. Still moreover, using the ionizing spray of ""418 independent of the snow spray and which is directed at an angle which is incident to the surface will further re-contaminate the substrate unless, as taught in ""793, the entire operation is performed in a controlled HEPA-filtered chamber.
As devices become smaller and their complexity increases, it is clearly desirable to have a improved processing technique, including a method and apparatus, that aids in using environmentally safe cleaning sprays to remove unwanted organic films and particles. It is desirable to have a technique that prevents additional particles and residues from being deposited on critical surfaces during application of said impingement cleaning sprays. The complete environmental control technique should include all of the basic environmental controls of thermal control, ionization control, and providing a dry and particle free cleaning atmosphere, but not negatively impacting the performance of the impinging cleaning spray. From the above, it is seen that a method and apparatus for use with impingement cleaning devices which provides micro-environmental control of precision electromechanical substrates during application that is low-cost, easy to use, adaptable and reliable is desired. As such, there is a present need to provide a method and apparatus for protecting a substrate from atmospheric contaminants and tribocharging effects during application of cryogenic cleaning sprays, and other non-cryogenic jet cleaning impingement sprays, which is low-cost, simple and adaptable to a variety of substrates and applicators. Moreover, there is a present need for an alternative and indirect environmental management process and apparatus whereas the spray applicator and other components are outside of the cleaning zonexe2x80x94thereby not posing a direct contamination threat to the critical substrate surfaces. Still moreover, there is a need for an environmental control process that does not produce a direct impingement spray upon the critical surfaces being cleaned and provides heat, ions and clean-dry atmosphere to the critical surfaces and indirectly removes contaminants discharged from the surfaces during spray cleaning operations. Finally, there is a need for an environmental control apparatus and method which can be used in any type of factory environmentxe2x80x94so as to not produce harmful contamination of adjacent factory assembly operations with particles and cleaning gases.
The present invention provides a low-cost, adaptive and selective method and apparatus to protect a device being subjected to a cryogenic impingement spray such as snow cleaning or dry steam spray. Examples include cleaning fiber optic connector end-faces, photodiodes, CCD""s and many other substrate cleaning applications. The present invention overcomes the limitations of conventional environmental control measures cited herein by providing a re-circulating localized microenvironment encompassing the entire critical substrate and holding fixturexe2x80x94a laminar sheath of temperature controlled, ionized inert dry gas which is delivered from behind the substrate surfaces being treated and which flows at an angle which is not incident to the substrate surface being treated. A relatively small partial pressure of clean dry air is continuously introduced into the cleaning cell to create a slight positive pressure within the cleaning cell, preventing the intrusion of ambient atmosphere. A localized exhaust plenum is used to extract the partial pressure of atmosphere escaping from the cleaning cell so as to prevent this atmosphere from diffusing into the local environmentxe2x80x94thus providing a balanced micro-environment using much less space, less clean air and less complex equipment than conventionally used to achieve the same results. The apparatus of the present invention comprises an open cell prophylacticxe2x80x94a protective device within which is contained a substrate and mounting fixture or which a substrate is placed therein, which provides an instantaneous curtain or sheath of controlled atmosphere. The prophylactic may be constructed of any variety of materials including metals, ceramics, glasses and conductive or ESD dissipative polymers, and combinations thereof, in which a cavity is created to accept the fixturexe2x80x9d and substrate. The cavity is selectively open to the ambient atmosphere and is ported to allow an inert pressurized and heated atmosphere (with respect to the ambient atmospherexe2x80x94outside the cavity), and flows within and envelops in a concurrent direction, in relation to the impinging spray, the entire fixture and substrate contained therein. The clean cavity atmosphere recirculates between the space comprising adjacent cavity wallsxe2x80x94the substrate thus being contained between the cavity walls and protected from ambient atmosphere in a sheath of clean air. Virtually any geometrically shaped cavity may formed about the surface and fixture to be treated. The substrate may be held within the purging cavity by means of a vacuum, manually held from above or held, articulated within the cavity by an external robot, or conveyed into the cleaning cell using an automated track. A re-circulated atmosphere, which may be ionized, flows at a controlled velocity in a manner consistent with the geometry of the cavity and substrate and fixture being treated to so as not to produce undue turbulence and erratic flow lines within the cavity. Thus the airflow may be circular, rectangular or any other shape as desired to form the appropriate flow patterns within the open cell cavity. Still moreover, the prophylactic may be designed to be interchangeable to accommodate any number of substrates and substrate geometries, such as reel-to-reel substrates, and may be integrated to the cryogenic spray applicator in such a manner as to allow for automatic placement over a substrate and performance of simultaneous spray cleaning operations.
The present invention provides both physical (structural) and chemical (ionic) ESD prevention and control componentsxe2x80x94a xe2x80x9cFaraday Cagexe2x80x9d of ions which surrounds and protects the substrate and fixture from electrical charges and radiation during spray treatments. Using exemplary coaxial TIG-Snow and TIG-Steam cleaning devices in combination with the present invention, both of which are described in detail within issued patents or pending patent applications, clean dry atmosphere is continuously introduced into the cleaning cell. As such, no significant additional clean atmosphere is required to make-up and maintain both the positive pressure and quality of the cleaning cell environment during cleaning operations.
Finally, the present invention provides a process and apparatus, which is adaptable to automated cleaning and assembly operations, and is a cost- and performance-effective alternative to fully-enclosed environmental enclosures. The present invention may be adapted to a robotic arm and integrated with the impingement cleaning spray applicator to provide automatic insertion, cleaning and treatment, and de-insertion of substratesxe2x80x94an in-situ ultraclean microenvironment for any type of production or assembly line.
Thus the present invention overcomes the limitations of prior art by providing an apparatus that uses less inert gas, is open architecture to allow for batch, in-line or online automation and manual cleaning operations, recirculates a majority of the clean atmosphere contained within the cleaning cell, is small enough to be used with any type of in-line automation system, is less complex and costly and can be configured in various physical forms to meet any batch or automated cleaning system design, and is a clean design and process that can be used within critical factory environments such as clean rooms.
A further understanding of the nature and advantages of the invention may be realized by reference to the latter portions of the specification and attached drawings.